1. Field of the Invention
The present invention relates to an environmental abnormality alarm apparatus for detecting a fire, gas leakage, burglary, or the like and generating an alarm.
2. Description of the Prior Art
So-called intelligence type alarm apparatuses are known wherein a common line extends from a central processing unit, a large number of terminals such as fire sensors or gas leakage sensors are connected to this line, the central processing unit sequentially circularly accesses the terminals by address polling, and analog values of the terminals are fetched and calculated, thereby generating an alarm and/or displaying an alarm content. In addition, some apparatuses have a fail-safe capability. That is, in these apparatuses, when the system is down or transmission abnormality of the central processing unit occurs, each terminal directly drives equipment to be controlled by a threshold value circuit provided in each terminal. An example of such an apparatus is described in Japanese Patent Publication (Kokai) No. 60-164802 (FIG. 5) filed by the present assignee. As shown in FIG. 5, a substation 40 functions as a terminal, and its detecting unit 30 detects smoke density, gas density, or the like caused by a fire as an analog value. When the substation 40 is accessed by a central processing unit 10, the analog value is A/D-converted into a digital code value and sent to the central processing unit 10, wherein the necessary calculations are undertaken. In addition, even if transmission is disabled by malfunction or the like of the central processing unit 10, a terminal 31 generates an alarm control output when an output from the detecting unit 30 exceeds a predetermined reference value E.sub.2.
A fail-safe output of the above conventional alarm apparatuses is controlled by comparing the analog value from the detecting unit 30 and the fixed second reference value E.sub.2. The reference value is changed by changing a resistance or a voltage generating element at the substation 40, resulting in a troublesome operation. In addition, variations are produced by temperature changes or deterioration over time. Furthermore, the reference value cannot be remotely changed from the central processing unit.